The need for high current drives in which thyristors (or SCR's) are natural and economic choices is particularly great in the driving of large electrical machines where high power requirements place significant demands on the line power. Typical of these applications are motors which drive rolling mills, such as those found in steel and aluminum plants, and the motors which drive the large fans and pumps used by utilities. To meet the requirement for drives for these types of large motors, cycloconverter circuits have been utilized. However, cycloconverters require a very large number of components (e.g., 72 thyristors), typically have undesirable input characteristics as seen by the power system, and provide undesirable harmonics in the output waveform, which requires additional front end converters.
Thyristor based load commutated inverters find important applications as variable speed drives in the 2,000 to 25,000 horsepower motor range. Natural commutation of the thyristors is dependent upon the inverter being presented with a leading power factor, typically obtained through excitation control of synchronous machine loads. However, at lower speeds, the synchronous machine reverts to a lagging power factor load, and natural commutation of the thyristors in the inverter is no longer possible. As a consequence, synchronous machine load commutated inverter drives are restricted in their frequency range and are difficult to start.
Special procedures and/or modifications to the converter circuit topology are required to allow the drive to operate below the lower frequency limits, particularly during start-up. One strategy uses a smaller rated auxiliary motor to bring the larger main motor up to a speed where load commutation of the inverter may be started. The load is subsequently connected to the larger motor after start-up. Another strategy uses firing angle control in the rectifier to interrupt the DC link current which supplies the load commutated inverter, allowing commutation to occur in the devices of the inverter. Performance using either of these strategies is characterized at low speeds by a quite limited torque capability and, particularly with the latter approach, by substantial torque ripple. As a practical matter, this restricts the loads normally driven by a load commutated inverter to those which require less torque at lower speeds and which rarely operate intentionally at low speed for very long. Examples of such loads are pumps and fans.
The load commutated inverter has also been applied to the driving of induction machines as loads, the leading power factor which is required being generated by additional capacitors connected in parallel with the load. As the frequency is reduced, the leading volt-amperes reactive (VARS) taken by the capacitors decreases until the power factor becomes lagging. Increasing the amount of capacitance is not a reasonable solution; doing so reduces the lower frequency limit, but does not eliminate it. Further, larger capacitances create leading VAR requirements which, at rated speed, can become unreasonably high. Capacitors also create resonances with the motor inductances, a problem which is difficult to control. Large terminal capacitors on an induction machine may cause an undesirable self-excitation under certain conditions, a problem which becomes progressively worse at higher speeds. Both of these approaches--excitation control and the adding of capacitance--, although widely used, have fundamental problems in at least three important performance aspects: speed range obtainable, torque obtainable at low speeds, in particular, and inverter starting procedure.
An alternate technique for extending the low frequency operating range of the load commutated inverter has been proposed in a paper by H. L. Hess and D. M. Divan, "A Method to Extend the Low Frequency Operation of Load Commutated Inverters," IEEE PESC Conference Record, June 1990, pp. 461-468. The circuit shown therein is configured as a normal load commutated inverter, but with small additional capacitors in parallel with the load. The inverter is switched at a frequency higher than the load fundamental so that the high frequency (switching frequency) current drawn by the capacitors is larger than the lagging current drawn by the load. Typically, the switching frequency is on the order of hundreds of hertz, limited by the switching losses. A net leading current is obtained for the inverter, allowing thyristor commutation, and permitting the lower frequency (load frequency) component to have a power factor which may be lagging, or even DC. As the synchronous machine speed increases to the point where it can supply its own leading VARS, the circuitry reverts to normal six step load commutated inverter operation. The control of the inverter was shown to be readily obtainable for single phase inverters. However, the strategy proposed for control of three phase inverters was complex, difficult to implement, and essentially incapable of controlling the system.